The present invention relates to improved gas distribution assemblies for use in fuel cells, to fuel cells employing such elements, and to processes for making such elements.
Reference is hereby made to other relates patent applications which are assigned to the same assignee as the present application; application of Charles J. Dettling and Peter L. Terry entitled "Integral Gas Seal For Fuel Cell Gas Distribution Assemblies and Method of Fabrication", Ser. No. 484,014, now U.S. Pat. No. 4,505,992 Filed Apr. 11, 1983, application of H. Feigenbaum and A. Kaufman entitled "Integral Gas Seal For Fuel Cell Gas Distribution Plate", Ser. No. 430,453, Filed on Sept. 30, 1982, application of H. Feigenbaum and S. Pudick entitled "A Process For Forming Integral Edge Seals in Porous Gas Distribution Plates Utilizing A Vibratory Means", Ser. No. 430,291, Filed on Sept. 30, 1982, and U.S. Pat. No. 4,450,212 of H. Feigenbaum, S. Pudick and R. Singh entitled "Edge Seal For Porous Gas Distribution Plate Of A Fuel Cell".
Fuel cell design and operation generally involves conversion of a hydrogen-containing fuel and some other oxidant via an exothermic reaction into D.C. electrical power. This reaction is well-known and has established parameters and limitations. It has been known for some time that fuel cells can be extremely advantageous as power supplies, particularly for certain applications such as a primary source of power in remote areas. It is highly desirable that any such cell assembly be extremely reliable. Various fuel cell systems have been devised in the past to accomplish these purposes. Illustrative of such prior art fuel cells are those shown and described in U.S. Pat. Nos. 3,709,736, 3,453,149 and 4,175,165. A detailed analysis of fuel cell technology comparing a number of different types of fuel cells appears in the "Energy Technology handbook" by Douglas M. Consadine, published in 1977 by McGraw Hill Book Company at pages 4-59 to 4-73.
U.S. Pat. No. 3,709,736, assigned to the assignee of the present invention, describes a fuel cell system which includes a stacked configuration comprising alternating fuel cell laminates and electrically and thermally conductive impervious cell plates. The laminates include fuel and oxygen electrodes on either side of an electrolyte comprising an immobilized acid. U.S. Pat. No. 3,453,149, assigned to the assignee of this invention, is illustrative of such an immobilized acid electrolyte. The fuel cells further comprise gas distribution plates, one in electrical contact with the anode and one in electrical contact with the cathode. The gas distribution plates conduct the reactant materials (fuel and oxidant) to the fuel cell.
In order to electrically interconnect a group of discrete cells to form one larger fuel cell stack, bipolar assemblies are commonly used. For instance, in U.S. Pat. No. 4,175,165, assigned to the assignee of the present invention, a stacked array of fuel cells is described wherein reactant gas distribution plates include a pluraltiy of gas flow channels or grooves for the distribution of the reactants. The grooves for the hydrogen gas distribution are arranged orthogonally relative to the grooves for the oxygen distribution.
The gas distribution plates themselves, whether they are part of termination assemblies having individual distribution plates for one or the other of the reactants or bipolar assemblies having two distribution plates for distributing both reactants in accordance with this disclosure, are formed of an electrically conductive impervious material. Where bipolar plates are prepared from a non-porous material, such as aluminum, the plate is typically coated with a layer of non-corrosive material, such as gold, so as to effectively isolate it from the corrosive agents, such a the electrolyte, within the fuel cell environment. In more recent fuel cell designs, the gas distribution plates of such assemblies are formed of a porous material so that a more uniform and complete flow of gas over the electrode surface is provided.
In previous systems wherein nonporous gas distribution plates were utilized, the reactants always flowed only through the grooves and were contained by the walls thereof. However, in the more recent systems utilizing porous plates, it has been necessary to seal the porous plates along the edges, and in bipolar assemblies, to segregate the reactants from one another to avoid their unintended mixing which could cause the cells to operate improperly or fail altogether.
Various techniques for sealing such porous gas distribution plates are known. For example, in aforementioned copending application Ser. No. 484,014, there is disclosed a porous bipolar gas distribution assembly, provided with an integral inner impervious region formed in two porous plates, preferably carbon, at the interface between the two plates by impregnating a sealant material therein. When impregnated into the porous plates, the sealant material acts as a bond to hold the plates together in a single integral bipolar assembly. Grooves are machined in the carbon plates on the outer facing surfaces opposite the interface layer, the grooves of one plate being substantially perpendicular to the grooves of the other plate. The impervious region prohibits reactant gases from mixing via through-plane transmission but permits electrical conductivity from plate to plate through the impervious region.
The bipolar gas distribution plate assemblies are fabricated by positioning a layer of sealant material between two porous plates and then simultaneously applying pressure and elevated temperature to the plates and layer of sealant material to melt the layer. The material in the layer impregnates the porous plates as it melts to bond the plates together. Through the proper selection of film thickness, pressure and temperature, the thermoplastic sealant film flows into the pores along the surface of each of the contiguous plates thereby effectively bonding one plate to the other and sealing each such plate along this common interface against gas transfer. Further, before the pressure on the bipolar assembly is removed, cooling is allowed to occur to a lower temperature.
In one embodiment of the disclosed process, a thermoplastic film of sealant, such as polyethersulphone, is sandwiched between two untreated porous carbon plates. This sandwich is placed in a hot-press, the temperature of the hot-press is elevated to heat the composite to a temperature in the range of approximately 500.degree.-700.degree. F., and the sandwich compressed under a pressure of approximately 200 to 500 psi. The temperature is maintained for a suitable period of time, such as 1/2 hour, and the length of the compression cycle is varied with the flow characteristics of the various sealant material. Subsequent to the completion of the compression cycle, the resultant sandwich is maintained under the compressive load within the press and cooled to ensure fusion of the lamina prior to release of pressure.
In completing the fabrication of the bipolar gas distribution assembly, it is necessary to seal the edges of the plates to prevent reactant gases from exiting through the plate edges and mixing together, for as already indicated, if leakage occurs, the cells could operate improperly or fail altogether.
Edge sealing of carbonaceous bipolar plates is normally done by coating the edges with a suspension of a sealant that leaves a continuous carbonaceous film or coating on the edge after the solvent has been evaporated. This is a labor-intensive operation and presents a hazard because of the noxious fumes released during drying of the applied edge coating.
Accordingly, the present invention provides an improved process for forming an edge seal in a bipolar gas distribution plate.
There is disclosed a process as above wherein the edge seal is formed simultaneously with the fabrication of the bipolar gas distribution assembly using a solid sealant compound thereby eliminating the need for a subsequent edge sealing step and avoiding the noxious fume problem.